The more the better: Polyandry in salamanders

For a long time, it was assumed that females in the animal world are monogamous, that is, they mate with only one male. Males, in contrast, can increase their reproductive success by mating with several females. Nowadays, however, polyandry is assumed to be the rule in the animal world and monogamy to be more of an exception.

Currently, researchers from completely different disciplines are interested in why females mate with several males and what benefits this brings for them or their offspring. There is a particular interest in studies that permit insights and conclusions on these processes under completely natural conditions. As a rule, however, such studies are hard to implement without disturbing the individuals or studying their mating behaviour completely or partially in the laboratory.

Researchers at Bielefeld University's Chair of Animal Behaviour in the group of Dr. Barbara Caspers, Dr. Sebastian Steinfartz, research group leader at the TU Braunschweig, and Professor Michael Kopp from Aix-Marseille University have studied the influence of mating behaviour on the number of offspring in the black and yellow fire salamander (Salamandra salamandra), a widespread European tailed amphibian species. Over the course of the spring season, a female salamander can deposit up to 50 living larvae in small streams and ponds. For their study, the scientists captured female salamanders on their way to deposit their larvae in a forest and took the pregnant females to the laboratory where they deposited their larvae. Every day, the scientists collected the new-born larvae, took a small tissue sample, and returned both mothers and their larvae to the forest. By subjecting these tissue samples to genetic paternity analyses, the researchers could precisely reconstruct how many males each female had mated with and whether or not the sperm of the different males had been mixed -- female salamanders can store the sperm of different males for several months in internal receptive organs called spermathecae. The eggs of the female will only be fertilized with the stored sperm, if environmental conditions are optimal and after eggs have developed into full larvae these are deposited in streams and ponds.

It is important to stress that the sperm from the male are deposited on the ground as a sperm packet (called spermatophore) during courtship display, and that the female then actively picks up the spermatophore. Afterwards, the female can decide from which males she will accept sperm or not. Through paternity analyses, the researchers were able to show that some females had mated with as many as four different males. The mixing of the sperm from various males in the spermatheca of the female seems to have quite positive effects, leading to more eggs being fertilized and, as a result, more larvae were finally deposited. Accordingly, polyandry and sperm competition seems to be an important mechanism to increase reproductive success and therefore fitness of a female in this terrestrial vertebrate species.